Process Synthesis (Process Creation)

Basic Chemicals

Ref: Product and Process Design Principles, Seider, Seader and Lewin, Chapter 3 in 2nd Ed, 4 in 3rd Ed.

The steps referred to as “process creation” are implemented by a design team when designing a process to manufacture a basic chemical product.

– It describes the components of the preliminary database

– It then shows how to create a synthesis tree with its many promising flowsheets.

– For each of the most promising alternative in the synthesis tree, a base-case design is created.

• Preliminary Database Creation – to assemble data to support the design.

• Experiments – often necessary to supply missing database items or

verify crucial data.

• Preliminary Process Synthesis – top-down approach.

– to generate a “synthesis tree” of design alternatives.

– illustrated by the synthesis of processes for the manufacture of VCM.

• Development of Base-case Design – focusing on the most promising alternative(s) from

the synthesis tree.

Preliminary Database Creation

• Thermophysical property data (for raw materials, desired products, by-products and reaction intermediates)

– physical properties (e.g. NBP, freezing point, critical properties, vapor pressures, heat capacities, formation energies etc.)

– phase equilibria (VLE data)

– property prediction methods

• Environmental and safety data (using MSDSs)

– toxicity data

– flammability data

• Chemical Prices

– e.g. as published in the Chemical Marketing Reporter

• Experiments

– to check on crucial items above

Additional data like transport properties, detailed chemical kinetics, data

for sizing, corrosivity of the chemicals are not necessary during process

creation.

Literature and Information Sources for Thermophysical Data

• National Laboratories and Research Institute Reports e.g. SRI International, NIST, NIOSH, DECHEMA

• Encyclopedias (technical, chemical process and technology).

• Handbooks and Reference Books (Perry’s Chemical Engineer’s Handbook, CRC Handbook of Physics and Chemistry)

• Journals (Book format, electronic format)

• Indexes (INSPEC, COMPENDEX, SCIENCE CITATION INDEX)

• Patents (U.S. Patent Office www.uspto.gov/patft )

• Databases provided by process simulators (ASPEN, UNISIM, CHEMCAD)

• Company context - employees, company files, open literature provide : Product info (related), thermophysical properties, transport data, flowsheets, equipment descriptions, process models.

• Auxiliary Studies (e.g. technical feasibility, marketing, business related)

Literature and Information Sources for Enviromental and Safety Data

• Material Safety Data Sheets

• US Environmental Protection Agency

http://java.epa.gov/chemview

• National Fire Protection Association (NFPA)

Preliminary Process Synthesis

The process design involves the synthesis of configurations that produce chemicals in a reliable, safe and economical manner, and at high yield with little or no waste.

Synthesis of chemical processes involves:

– Selection of processing mode: continuous or batch (the scale of the process is a primary consideration)

– Fixing the chemical state of raw materials, products, and by-products

– Process operations (unit operations) - flowsheet building blocks

– Synthesis steps to eliminate the differences

– Heuristics

Continuous or batch processing?

Continuous

Batch

Fed-batch

Batch-product removal

The Chemical State

Decide on the raw material and product specifications (states):

• Mass (flow rate)

• Composition (mole or mass fraction of each chemical species having a unique molecular type)

• Phase (solid, liquid, or gas)

• Form (e.g., particle-size distribution and particle shape)

• Temperature

• Pressure

Process Operations

• Chemical reaction

• Separation of chemicals

• Phase separation

• Change of temperature

• Change of pressure

• Change of phase

• Mixing and splitting of streams and branches

• Chemical reaction operations are at the heart of many chemical processes. Positioning in the flowsheet involves many considerations (conversion, rates, etc.), related to T and P at which the reaction are carried out.

• Separation of chemicals appear in almost every process flowsheet. They are needed to resolve difference between the desired composition of a product stream and that of its source. Selection of the appropriate method depends on the differences of the physical properties of the chemical species involved.

• Phase separation: Many separation operations require phase separation operations, which may be accomplished by vessels like flash drums for vapor-liquid separation, decanters for liquid-liquid separations

• Change of temperature: often a process stream needs to be heated or cooled from its source temperature to target temperature, which is best accomplished by heat exchange. In process synthesis, heat exchangers are inserted into the flowsheet to satisfy heating and cooling demands and a few of the concepts associated with heat integration are introduced.

• Change of pressure: It is common to select the pressure levels for reaction and separation operations. Then pressure-change operations will be necessary, however, those operations ,e,g. compressors, pumps, turbines, often ignored in the early stages of process design.

• Change of phase: It is common to position a phase change operation using temperature and/or pressure-change operations.

• Mixing and splitting of streams and branches: Mixing operation is necessary to combine two or more streams and is insered when chemicals are recycled or when it is necessary to blend two or more streams to achieve a product specification. Splitting is necessary if two or more streams of the same temperature, pressure and composition is required.

Synthesis Steps

Synthesis Step

1. Eliminate differences in molecular types

2. Distribute chemicals by matching sources and sinks

3. Eliminate differences in composition

4. Eliminate differences in temperature, pressure and phase

5. Integrate tasks (combine tasks into unit operations)

Process Operation

Chemical reaction

Mixing

Separation

Temperature, pressure and phase change

Example: Vinyl Chloride Manufacturing

To satisfy the need for an additional 800 MMlb/yr of VCM, the following plausible alternatives might be generated: Alternative 1. A competitor’s plant, which produces 2 MMM lb/yr of VCM and

is located about 100 miles away, might be expanded to produce the required amount, which would be shipped. In this case, the design team projects the purchase price and designs storage facilities.

Alternative 2. Purchase and ship, by pipeline from a nearby plant, chlorine from

the electrolysis of NaCl solution. React the chlorine with ethylene to produce the monomer and HCl as a byproduct.

Alternative 3. Since the existing company produces HCl as a byproduct in large

quantities are produced, HCl is normally available at low prices. Reactions of HCl with acetylene, or ethylene and oxygen, could produce 1,2-dichloroethane, an intermediate that can be cracked to produce vinyl chloride.

1. Eliminate differences in molecular types

Chemical Molecular

weight

Chemical

formula

Chemical

structure

Acetylene 26.04 C2H2 H - C C - H

Chlorine 70.91 Cl2 Cl-Cl

1,2-Dichloroethane

98.96

C2H4Cl2

Cl Cl

| |

H-C-C-H

| |

H H

Ethylene

28.05

C2H4

H H

C = C

H H

Hydrogen chloride 36.46 HCl H-Cl

Vinyl chloride

62.50

C2H3Cl

H Cl

C = C

H H

Chemicals participating in VC Manufacture:

a. Direct chlorination of ethylene:

Selection of pathway to VCM (1)

Advantages:

– Attractive solution to the specific problem denoted as Alternative 2 in analysis of primitive problem.

– Occurs spontaneously at a few hundred oC.

Disadvantages:

– Does not give a high yield of VC without simultaneously producing large amounts of by-products such as dichloroethylene

– Half of the expensive chlorine is consumed to produce HCl by-product,

which may not be sold easily.

HCl ClHC Cl HC 32242

2. Hydrochlorination of acetylene:

Advantages:

– This exothermic reaction is a potential solution for the specific problem

denoted as Alternative 3. It provides a good conversion (98%) of C2H2 VC

in the presence of HgCl2 catalyst impregnated in activated carbon at

atmospheric pressure.

– These are fairly moderate reaction conditions, and hence, this reaction deserves further study.

Disadvantages:

– Flammability limits of C2H2 (2.5 100%)

ClHC HCl HC 3222

3. Thermal cracking of C2H4Cl2 from chlorination of C2H4:

Advantages:

– Conversion of ethylene to 1,2-dichloroethane in exothermic reaction is

98% at 90 oC and 1 atm with a Friedel-Crafts catalyst such as FeCl3. This

intermediate is converted to vinyl chloride by thermal cracking according to

the endothermic reaction (2.4), which occurs spontaneously at 500 oC with

conversions as high as 65% (Alternative 2).

Disadvantage:

– Half of the expensive chlorine is consumed to produce HCl by-product,

which may not be sold easily.

242242 ClHC Cl HC

HCl ClHC ClHC 32242

HCl ClHC Cl HC 32242

4. Thermal Cracking of C2H4Cl2 from Oxychlorination of C2H4:

Advantages:

– Highly exothermic reaction (2.5) achieves a 95% conversion to C2H4Cl2 in

the presence of CuCl2 catalyst, followed by pyrolysis step (2.4) as

Reaction Path 3.

– Excellent candidate when cost of HCl is low

– Solution for specific problem denoted as Alternative 3.

Disadvantages:

– Economics dependent on cost of HCl

OH ClHC O 2HCl HC 22422 2

1

42

HCl ClHC ClHC 32242

OH ClHC O HCl HC 2322 2

1

42

5. Balanced Process for Chlorination of Ethylene:

Advantages:

– Combination of Reaction Paths 3 and 4 - addresses Alternative 2.

– All Cl2 converted to VC

– No by-products!

OH ClHC O 2HCl HC 22422 2

1

42

2HCl ClHC2 ClH2C 32242

242242 ClHC Cl HC

OH ClHC2 O Cl HC2 2 3222

1

242

Evaluation of Alternative Pathways

Chemical Cost (cents/lb)

Ethylene 30

Acetylene 80

Chlorine 18

Vinyl chloride 35

Hydrogen chloride 25

Water 0

Oxygen (air) 0

Chemical Bulk Prices

• Reaction Path 1 is eliminated due its low selectivity.

• This leaves four alternative paths, to be compared first in terms of Gross Profit.

Computing Gross Profit

Reaction path 3 C2H4 + Cl2 = C2H3Cl + HCl

lb-mole 1 1 1 1

Molecular weight 28.05 70.91 62.50 36.46

Amount (lb) 28.05 70.91 62.50 36.46

lb/lb of vinyl chloride 0.449 1.134 1 0.583

cents/lb 30 18 35 25

Gross profit = 35(1) + 25(0.583) - 30(0.449) - 18(1.134) = 15.69 cents/lb VC

Reaction Path

Overall Reaction Gross Profit

(cents/lb of VC)

2 C2H2 + HCl = C2H3Cl -16.00

3 C2H4 +Cl2 = C2H3Cl + HCl 15.69

4 C2H4 + HCl + O2 = C2H3Cl + H2O 6.96

5 2C2H4 + Cl2 + O2 = 2C2H3Cl + H2O 11.32

Raw Materials

Process Flowsheet?

C2H4, Cl2

Products

C2H3Cl, HCl

Cl2

113,400 lb/hr

C2H4

44,900 lb/hr

Direct

ChlorinationPyrolysis

C2H4Cl2

HCl

58,300 lb/hr

C2H3Cl

100,000 lb/hr

HCl

C2H3Cl

C2H4Cl2

C2H4Cl2 C2H3Cl + HClC2H4 + Cl2 C2H4Cl2

Preliminary Flowsheet for Path 3

• 800 MM lb/year @ 330 days/y 100,000 lb/hr VC

• On the basis of this principal sink, the HCl sink and reagent sources can be computed (each flow is 1,600 lbmol/h)

• Next step involves distributing the chemicals by matching sources and sinks.

2. Distribute the chemicals

• A conversion of 100% of the C2H4 is assumed in the chlorination reaction.

• Only 60% of the C2H4Cl2 is converted to C2H3Cl with a byproduct of HCl,

according to Eqn. (2.4).

• To satisfy the overall material balance, 158,300 lb/h of C2H4Cl must

produce 100,000 lb/h of C2H3Cl and 58,300 lb/h of HCl.

• But a 60% conversion only produces 60,000 lb/h of VC.

• The additional C2H4Cl2 needed is computed by mass balance to equal:

[(1 - 0.6)/0.6] x 158,300 or 105,500 lb/h.

• Its source is a recycle stream from the separation of C2H3Cl from

unreacted C2H4Cl2, from a mixing operation, inserted to combine the

two sources, to give a total 263,800 lb/h.

• The effluent stream from the pyrolysis operation is the source for the C2H3Cl product, the HCl by-product, and the C2H4Cl2 recycle.

• Reactor pressure levels:

– Chlorination reaction: 1.5 atm is recommended, to eliminate the possibility of an air leak into the reactor containing ethylene.

– Pyrolysis reaction: 26 atm is recommended by the B.F. Goodrich patent (1963) without any justification. Since the reaction is irreversible, the elevated pressure does not adversely affect the conversion. Most likely, the patent recommends this pressure to reduce the size of the pyrolysis furnace, although the tube walls must be considerably thicker and many precautions are necessary for operation at elevated pressures.

– The pressure level is also an important consideration in selecting the separation operations, as will be discussed in the next synthesis step.

• The product of the chlorination reaction is nearly pure C2H4Cl2, and requires no purification.

• In contrast, the pyrolysis reactor conversion is only 60%, and one or more separation operations are required to match the required purities in the C2H3Cl and HCl sinks.

• One possible arrangement is given in the next slide. The data below explains the design decisions made.

3. Eliminate Differences in Composition

Boiling point (oC) Critical constants

Chemical 1 atm 4.8 atm 12 atm 26 atm Tc,C Pc, atm

HCl -84.8 -51.7 -26.2 0 51.4 82.1

C2H3Cl -13.8 33.1 70.5 110 159 56

C2H4Cl2 83.7 146 193 242 250 50

Boiling point (oC) Critical constants

Chemical 1 atm 4.8 atm 12 atm 26 atm Tc,C Pc, atm

HCl -84.8 -51.7 -26.2 0 51.4 82.1

C2H3Cl -13.8 33.1 70.5 110 159 56

C2H4Cl2 83.7 146 193 242 250 50

4. Eliminate differences in T, P and phase

5. Integrate tasks (tasks unit operations)

Synthesis Tree

Development of Base-case Design

Develop one or two of the more promising flowsheets from the synthesis tree for more detailed consideration.